The present invention relates to an electro-optical distance meter, which projects a modulated measuring light to an object to be measured, receives a reflected measuring light from the object to be measured and measures a distance to the object to be measured by a phase difference between the measuring light and the reflected measuring light.
In the electro-optical distance meter which perform a distance measurement by detecting the phase difference between the measuring light and the reflected measuring light, a distance as measurable and a measurement accuracy are determined by a modulated frequency. For this reason, in order to make measuring from a near distance to a long distance possible, it is necessary to project a distance measuring light with a plurality of modulated frequencies.
For instance, modulated frequency of 30 MHz, a modulated frequency of 300 KHz, and further, frequencies of 30 MHz-3 KHz and 300 KHz-3 KHz are prepared. Distance measuring lights are emitted each with modulated frequency of 30 MHz and a modulated frequency of 300 KHz respectively, and reflected distance measuring lights having a modulated frequency of 30 MHz and a modulated frequency of 300 KHz are received, and further, frequencies of 30 MHz-3 KHz and 300 KHz-3 KHz from photodetection signals are mixed. Thereby, it is possible to perform an electro-optical distance measurement from a near distance to a long distance by detecting a difference frequency signal of 3 KHz and by detecting a phase difference respectively.
In a conventional electro-optical distance meter, since a plurality of frequencies are prepared, distance measuring light is projected per each plurality of frequencies, and distance measurement is performed, there have been problems in that a circuit arrangement becomes complicated and measurement time becomes longer.